The Cardiovascular System

Heart Anatomy and Structural Organization

• Location: Hollow muscular organ situated in the mediastinum of the thoracic cavity between the lungs, positioned slightly left of the midline and enclosed within the pericardial sac.

• Size and Weight: Approximately the size of a fist ($12\,\text{cm}$ long, $9\,\text{cm}$ wide, $6\,\text{cm}$ thick) with an adult weight of $250\text{--}350\,\text{g}$.

• Surface Landmarks:

  • Base: Broad superior region situated at the level of the 3rd costal cartilage.

  • Apex: Pointed inferior region formed by the left ventricle, located left of the sternum between the 4th and 5th intercostal spaces (site of the apical pulse; creates the cardiac notch in the left lung).

• Cardiopulmonary Resuscitation (CPR) Relevance: Positioned directly behind the sternum and anterior to the vertebral column; external compressions squeeze the heart against the spine to generate intrathoracic pressure and circulate blood.

• Heart Wall Layers:

  • Pericardium: Tough outer fibrous pericardium anchoring the heart, plus an inner serous pericardium with two layers (parietal layer lining the fibrous sac and visceral layer covering the heart surface), separated by a lubricating serous fluid that prevents friction and overfilling.

  • Epicardium: Outermost layer of the heart wall, identical to the visceral layer of the serous pericardium.

  • Myocardium: Middle, thickest layer composed of contractile cardiomyocytes that generate pumping force.

  • Endocardium: Deepest layer lining internal chambers and valves, made of endothelial tissue.

Heart Chambers, Valves, and Circulation

• Internal Chambers:

  • Right Atrium: Collects deoxygenated blood from systemic circulation through the superior vena cava and inferior vena cava.

  • Right Ventricle: Expels deoxygenated blood into the pulmonary circuit toward the lungs.

  • Left Atrium: Receives newly oxygenated blood returning from the lungs via four pulmonary veins.

  • Left Ventricle: Thickest muscular chamber; discharges oxygenated blood into systemic circulation under high pressure via the aorta.

• Cardiac Valves:

  • Atrioventricular (AV) Valves: Prevent backflow from ventricles into atria during systole; anchored by chordae tendineae and papillary muscles. Consist of the Tricuspid valve (right side, 3 leaflets) and Mitral/Bicuspid valve (left side, 2 cusps).

  • Semilunar Valves: Prevent backflow from outflow vessels into ventricles during diastole. Consist of the Pulmonary valve (at the base of the pulmonary trunk, 3 flaps) and Aortic valve (at the base of the aorta, 3 flaps).

• Blood Flow Pathway:

  • $\text{Body} \rightarrow \text{SVC/IVC} \rightarrow \text{Right Atrium} \rightarrow \text{Tricuspid Valve} \rightarrow \text{Right Ventricle} \rightarrow \text{Pulmonary Valve} \rightarrow \text{Pulmonary Arteries} \rightarrow \text{Lungs}$

  • $\text{Lungs} \rightarrow \text{Pulmonary Veins} \rightarrow \text{Left Atrium} \rightarrow \text{Mitral Valve} \rightarrow \text{Left Ventricle} \rightarrow \text{Aortic Valve} \rightarrow \text{Aorta} \rightarrow \text{Systemic Circulation}$

• Coronary Circulation:

  • Originates directly above the aortic valve cusps from the aortic root.

  • Left Coronary Artery (LCA): Divides into the circumflex artery and left anterior descending artery (LAD) / anterior interventricular artery; supplies the left atrium, left ventricle, and interventricular septum.

  • Right Coronary Artery (RCA): Divides into marginal branches and the posterior interventricular artery (PDA); supplies the right atrium, conduction nodes, and portions of both ventricles.

Conduction System, Cardiac Cycle, and Heart Sounds

• Cardiomyocytes: Involuntary, striated, branching cells connected by intercalated discs; composed of $99\%$ contractile cells (generate pumping force) and $1\%$ specialized conducting cells (initiate and propagate action potentials).

• Electrical Conduction Pathway:

  • Sinoatrial (SA) Node: Primary pacemaker situated in the superior wall of the right atrium ($60\text{--}100\,\text{bpm}$; distributes signal across atria and via Bachmann's bundle to the left atrium).

  • Atrioventricular (AV) Node: Located in the inferior interatrial septum; slows electrical conduction to permit complete atrial emptying into the ventricles.

  • Bundle of His (Atrioventricular Bundle): Directs impulses through the interventricular septum, bifurcating into right and left bundle branches.

  • Purkinje Fibers: Transmit impulses rapidly across ventricular walls from the apex toward the base to initiate ventricular contraction.

• Phases of the Cardiac Cycle:

  • Atrial Systole: Atrial contraction forces the final $20\text{--}30\%$ of ventricular filling ($\sim 100\,\text{ms}$).

  • Ventricular Systole: Divided into isovolumic contraction (ventricular pressure rises with all valves closed) and ventricular ejection (blood pumped past open semilunar valves; stroke volume $\sim 70\text{--}80\,\text{mL}$, residual end-systolic volume $\sim 50\text{--}60\,\text{mL}$).

  • Ventricular Diastole: Divided into isovolumic relaxation (pressure plummets as semilunar valves close) and late passive ventricular filling as AV valves open.

• Heart Sounds:

  • $S_1$ ("lub"): Sound produced by the closure of AV valves during ventricular systole.

  • $S_2$ ("dub"): Sound produced by the closure of semilunar valves during ventricular diastole.

  • Abnormal Sounds: $S_3$ ("Kentucky gallop", rare in health, sign of heart failure in older adults); $S_4$ ("Tennessee gallop", forceful atrial contraction into a stiff ventricle, indicates left ventricular failure if heard before $S_1$); $S_7$ (combined presence of $S_3$ and $S_4$); murmurs (turbulent flow graded on a $1\text{--}6$ severity scale).

Hemodynamics and Cardiovascular Regulation

• Cardiac Output ($\text{CO}$):

  • $\text{CO} = \text{HR} \times \text{SV}$

  • Heart Rate ($\text{HR}$): Normal resting frequency is $60\text{--}100\,\text{bpm}$ (average $\sim 75\,\text{bpm}$; controlled by nerves and hormones).

  • Stroke Volume ($\text{SV}$): Volume pumped per contraction; $\text{SV} = \text{EDV} - \text{ESV}$ (normal range $55\text{--}100\,\text{mL}$, typical average $\sim 70\,\text{mL}$).

• Blood Pressure ($\text{BP}$):

  • Governed by Cardiac Output ($\text{CO}$) and Systemic Vascular Resistance ($\text{SVR}$).

  • Systolic BP: Peak pressure within arteries during ventricular ejection.

  • Diastolic BP: Lowest pressure within arteries during ventricular relaxation.

  • Pulse Pressure: Calculated as $\text{Systolic BP} - \text{Diastolic BP}$.

• Baroreceptor Reflex (Short-Term Control):

  • High-pressure stretch receptors are located in the carotid sinuses and aortic arch; low-pressure receptors reside in the atria and pulmonary vessels.

  • A drop in arterial blood pressure decreases receptor stretch and afferent signaling to the medulla oblongata.

  • The medullary cardiovascular center compensates by increasing sympathetic activity (raising heart rate, contractility, and inducing arteriolar vasoconstriction) and decreasing parasympathetic vagal output.

• Factors Influencing Heart Rate and Contractility:

  • Increase: Sympathetic stimulation (norepinephrine), exercise (proprioceptors), low $\text{O}_2$ / high $\text{CO}_2$ / high $\text{H}^+$ (chemoreceptors), decreased baroreceptor firing, catecholamines (epinephrine, norepinephrine), thyroid hormones ($T_3$, $T_4$), elevated $\text{Ca}^{2+}$, decreased $\text{K}^+$, decreased $\text{Na}^+$, hyperthermia, caffeine, nicotine.

  • Decrease: Parasympathetic stimulation (acetylcholine via vagus nerve), post-exercise rest, high $\text{O}_2$ / low $\text{CO}_2$ / low $\text{H}^+$, high blood pressure (increased baroreceptor firing), decreased $\text{Ca}^{2+}$, elevated $\text{K}^+$, elevated $\text{Na}^+$, hypothermia.

Vascular System and Major Vessels

• Vessel Wall Architecture (Tunics):

  • Tunica Intima: Innermost endothelial lining and basement membrane.

  • Tunica Media: Middle layer of smooth muscle and elastic fibers; thicker in arteries to manage high pressures.

  • Tunica Externa (Adventitia): Outermost connective tissue sheath containing the vasa vasorum and nervi vasorum.

• Comparison of Blood Vessels:

  • Arteries: Thick muscular walls, small rounded lumens, carry blood under high pressure away from the heart; lack valves.

  • Arterioles: Small arterial branches with smooth muscle rings; primary regulators of systemic vascular resistance and blood pressure.

  • Capillaries: Microscopic, single-endothelial-cell layer thick; primary sites of nutrient, gas, and waste exchange.

  • Venules: Small vessels collecting blood from capillary beds.

  • Veins: Thin-walled, large irregular lumens, low pressure; equipped with unidirectional valves (chiefly in limbs and inferior to the heart) to prevent venous backflow.

• Anatomical Arterial Pulse Palpation Sites: Superficial temporal, facial, common carotid, brachial, radial, femoral, popliteal, posterior tibial, and dorsalis pedis arteries.

• Aortic Divisions and Branches:

  • Ascending Aorta: Extends upward approximately $5\,\text{cm}$ from the left ventricle.

  • Aortic Arch: Curves laterally and terminates near the T4–T5 vertebrae. Gives rise to 3 principal vessels: Brachiocephalic artery (branches into right common carotid and right subclavian arteries), Left common carotid artery, and Left subclavian artery.

  • Descending Aorta: Subdivided into the Thoracic aorta (above the diaphragm) and Abdominal aorta (below the diaphragm; bifurcates at L4 into right and left common iliac arteries).

• Cerebral Arterial Supply: Internal carotid and vertebral arteries merge at the base of the brain to form the arterial Circle of Willis, ensuring continuous cerebral perfusion.